A frequently-encountered problem in industrial inspection and quality control is the precise measurement of small distances between surfaces. In magnetic data storage systems, for example, it is required to measure the flying height of a slider assembly near contact on a rapidly rotating rigid disk in order to verify the performance of the slider assembly. The flying height, as used herein, is the distance between the magnetic head pole and the surface of the rotating rigid disk; see, e.g., M. F. Garnier, et. al., U.S. Pat. No. 3,855,625 issued Dec. 17, 1974. The flying results from the aerodynamic effects produced by the rigid disk's rotation. The flying heights are generally less than 250 nm (10.mu.-inch) depending on the design of the slider, and may be as close as a few tens of nanometers. The trend in the art is toward very low flying heights, that is, less than 25 nanometers. The speed and reliability of the measurement is of particular importance, since a single slider manufacturer typically produces 200,000-500,000 slider assemblies per month.
Prior-art apparatus and methods for measuring the flying height of a slider assembly are disclosed in B. Bhushan, Tribology and Mechanics of Magnetic Storage Devices, pp. 765-797 (New York: Springer-Verlag, 1990). Some more recent developments are described in "Proceedings of the IDEMA Sub 2-micro inch Workshop", May 12, 1993.
Optical flying-height testers (OFHT's) are almost invariably based on interferometry. Interferometers are capable of determining the distance to an object, the topography of the object, or like physical parameters involving physical lengths (see, for example, Chapter 1 of the book Optical Shop Testing, second edition, edited by Daniel Malacara (Wiley, New York, 1992). One of the fundamental difficulties of optical techniques is that the interface between the slider ABS and a real hard disk cannot be inspected directly. Therefore, there are essentially two different types of OFHT's, those which perform a relative measurement the back side of the slider flying on a real disk, and those that use a transparent glass surrogate in place of a real hard disk.
An example of the first kind of OFHT is provided in an article entitled "Measurement of head/disk spacing with a laser interferometer," by L.-Y. Zhu, K. F. Hallamasek, and D. B. Bogy (IEEE Tran. Magn., MAG-23, 2739, 1988). The disclosed apparatus is a heterodyne interferometer capable of measuring the physical position of a plurality of points on the back side of a slider, that is, points on the side of the slider that is not in near contact with the disk. The advantages of this apparatus are that it functions with a real magnetic hard disk, and it is capable of measuring the orientation (pitch and roll) as well as the height of the slider in flight. Calibration for zero flying height is performed by landing the slider on the disk. The principle disadvantage of this kind of system is that the slider/disk interface is not observed directly, and the flying height can only be inferred from the position of the back side of the slider. Thus it must be assumed that the slider thickness and ABS shape are constant, while in fact there may be significant distortions of the slider due to mechanical and thermal stress during flight. Another disadvantage is that the back of the slider is currently not accessible on most production slider assemblies.
The first reported direct measurement of the slider/disk interface by interferometric means was reported by W. Stone in an article entitled "A proposed method for solving some problems in lubrication" (The Commonwealth Engineer, November, 1921 and December, 1921). Stone was obviously not working with magnetic storage media in 1921, but the essential concepts are the same ones that underlay the majority of modern OFHT's. Stone's apparatus comprises a glass disc about 125 mm in diameter so mounted that it can be rotated in a horizontal plane. A 15 mm-square block, substantially similar in function to a slider, is pressed against the lower surface of the disc by means of a suitable loading mechanism. Since the disk is transparent, it is possible to view the block through the disk while it is in flight. The block is illuminated through the disk with a sodium flame, which for the intended purpose acts as a nearly monochromatic, unpolarized light source. The reflected beam is composed of a combination of the light beam reflecting from the surface of the disk and the light beam reflecting from the block. The combination and simultaneous detection of these two reflected beams results in an interference effect related to the flying height of the block above the disk. The spacing between the block and the disk as well as the orientation of the block is deduced by visual inspection of the interference pattern as the disk varies in speed.
Modern commercial OFHT's that measure the slider/disk interface directly are based on many of the same physical principles as the apparatus invented by Stone, with the differences being principally in the type of source, detector and data processing means. A transparent surrogate disk replaces the magnetic hard disk and the interference effects at the slider ABS provide the flying height information. All prior-art OFHT's of this type illuminate the ABS through the disk at substantially normal incidence, and detect the variations in reflected intensity irrespective of polarization.
One form of OFHT uses a substantially monochromatic source light, as disclosed for example by G. L. Best, D. E. Horne, A. Chiou and H. Sussner, in a paper entitled "Precise optical measurement of slider dynamics," IEEE Trans. Magn. MAG-22, (1986) 1017-1019. The reflected light is modulated by the thin-film effect between the disk and the slider ABS. This modulation is periodic with the flying height, and has a period equal to one-half the wavelength of the source illumination. By introducing appropriate detection and analysis means, it is possible to track variations in the flying height by observing the modulations in intensity of the reflected light. Over certain portions of the modulation curve, it is possible by detection of the reflected intensity to determine the gap between the ABS and the disk with reasonable accuracy. Originally, such instruments involved a purely visual interpretation of the fringes. J. M. Fleischer and C. Lin were the first to use a photo-electric sensor in a monochromatic OFHT, as is described in an article entitled "Infrared laser interferometer for measuring air-bearing separation," (IBM Journal of Research and Development, 18(6), 1974, pp.529-533). A more modern example of monochromatic OFHT is described by T. Ohkubo and J. Kishegami in an article entitled "Accurate Measurement of Gas- Lubricated Slider Bearing Separation using Laser Interferometry," Trans. ASME, Vol 110, pp148-155 (January, 1988). This article describes the basis of operation for the FM8801 and FM2000 Fly Height Testers sold in the U.S.A. by ProQuip, Inc.
In that the measurement depends on a periodic phenomenon, a disadvantage of the monochromatic OFHT is that it is not clear which interference cycle is being measured. There is consequently an ambiguity and the flying-height measurement is restricted to a range equal to one-quarter of the wavelength, i.e. typically less than 150 nm. A further difficulty is that there are significant ranges of flying height over which the sensitivity of the measurement is nearly zero. This aspect of the measurement method is particularly troublesome when the gap between the slider and the disk is less than 25 nm Finally, it may be necessary to land every slider in a production test to calibrate the system for zero flying height.
In the paper entitled "A Visible Laser Interferometer for Air Bearing Separation measurement to Submicron Accuracy," by A. Niagam, Trans. ASME, Vol. 104, pp. 60-65 (1982) there is described an OFHT based on monochromatic light which also provides additional means of determining the interferometric fringe order. The additional means comprise a Xenon lamp and a circular variable wavelength filter. The lamp and wavelength filter function together as a tunable wavelength source with a range of 400 to 700 nm. As the wavelength is shifted, the interference pattern is also shifted in a way which reveals the absolute flying height and thus the fringe order for the monochromatic measurement. Once the fringe order has been determined, the measurement proceeds with the monochromatic sensor at a rate of approximately 2.5 kHz.
Several other prior-art systems avoid the ambiguity problems of monochromatic interferometry by including multiple wavelengths. For example, a common form of OFHT is based on the variation of the interference effect with the wavelength of the illumination, as is taught for example in the U.S. Pat. No. 4,593,368 to D. A. Fridge, et al. The apparatus in this patent comprises a computerized spectro-photometer, which analyzes the wavelength-dependent modulation of white light reflected from the slider-disk interface. This technique is incorporated in commercially available products such as the line of Automatic Digital Flying Height Testers formerly produced by Pacific Precision Laboratories, Inc. (PPL) of Chatsworth, Calif. White light interferometry has the significant advantage that there is no ambiguity in the measurement, since the spectral modulation phenomenon is not periodic with flying height. However, white light methods based on spectrometers suffer from a number of limitations, the most severe and intractable limitation being the measurement speed. This problem is compounded by the need to compensate for the phase change on reflection for as many as 171 different wavelengths (see, for example, an article entitled "Flying height measurement systems and slider absorption", by R. Pavlat, IDEMA Insight 7(5), p.1 (1994)). Finally, white light techniques are most effective for gaps greater than one-half the wavelength of the shortest wavelength used, i.e., approximately 200 nanometers, whereas the trend is towards flying heights of less than 25 nm.
In order to overcome some of the limitations of white light interferometry mentioned above, several prior-art OFHT's use a small number of discrete wavelengths of light to improve speed and performance. In the U.S. Pat. No. 5,280,340 to C. Lacey there is described a three-wavelength method of optically analyzing small spacings that comprises a high-intensity source of multiple-wavelength radiation and a detector assembly for rapid spectral analysis. The detector assembly includes wavelength discriminating beamsplitters, a filter for each individual wavelength to be measured and a high speed photodetector for each wavelength. The disclosed apparatus also comprises a mechanical assembly which is used to move the head away from the detection assembly a very small distance, on the order of 0.25 .mu.m. This mechanism is required for calibration of the apparatus, which involves measurement of the intensity of two or more wavelengths while partially unloading the slider to determine the maximum and minimum intensity at each wavelength. Once the system is calibrated, it is capable of measuring flying heights at rates greater than 100 kHz. The apparatus disclosed in this patent is the basis of the Dynamic Flying Height Tester manufactured by Phase Metrics.
Although three-wavelength OFHT's are much faster than older white-light instruments, they still share many of the same limitations, the most serious of which is that the measurement sensitivity approaches zero as the flying height approaches zero. These limitations are related principally to the reliance on the variation in reflected intensity at normal incidence for a range of wavelengths. These variations can be extremely difficult to measure when the flying height is small. Therefore the reliance on intensity measurements at normal incidence is a fundamental deficiency of all of the prior-art OFHT's cited above.
The measurement difficulty at low flying heights is largely avoided if the reflection from the slider ABS and the reflection from the disk surface can be separated in some way, either by polarization, physical separation of the beams, or both. The apparatus disclosed in commonly-owned U.S. Pat. No. 4,606,638 to G. Sommargren uses a transparent disk as a front surface polarizer, so that the reflection from this surface can be distinguished from the reflection from the ABS. An additional advantage of the disclosed apparatus is that the entire gap is measured by a camera having a plurality of detectors, thus making it possible to determine the shape and orientation of the slider, as well as other parameters of interest that require a plurality of measurement points. However, the manufacture of the special transparent disk with the polarization coating, as taught in the Sommargren patent, is very costly and any surface imperfections can cause problems at low flying height.
Another approach to separating the interfering beams in an optical flying-height tester is disclosed in commonly-owned U.S. Pat. No. 5,218,424 to G. Sommargren. The apparatus uses two parallel beams having orthogonal polarizations. Both beams are incident on the surface of the glass disk at Brewster's angle, so that one of the beams passes completely through the disk without reflection, and the other is partially reflected from the surfaces of the disk. The beam that passes through the disk without reflection is used to illuminate the ABS The two beams are then recombined, resulting in an interference effect that varies sinusoidally with the flying height. Since the apparatus taught in this patent is a two-beam interferometer, it is possible to measure extremely small gaps without loss of sensitivity and precision, thus eliminating one of the principle disadvantageous of systems that depend on interference effects resulting directly from multiple reflections within the gap. The disclosed apparatus also comprises an array camera for imaging the entire ABS.
Despite these advantages, the method and apparatus disclosed in commonly-owned U.S. Pat. No. 5,218,424 has significant limitations that make it an impractical tool for automated inspection of the flight characteristics of sliders used in the magnetic storage industry. These limitations include the use of an expensive, complicated, high-speed phase modulator as an essential component; a very slow data acquisition and processing rate of approximately 15 Hz, which is due in part to the method of phase modulation and the need to integrate over a full rotation of the transparent disk; a very slow determination of the dynamic flight characteristics of the slider, which is due in part to the use of a full-frame imaging camera for all measurements; a deleterious sensitivity to inhomegeneaties and distortions in the transparent disk; a deleterious sensitivity to the tip and tilt of the disk, which can introduce substantial errors in the flying height measurement; and an overall drift in the interference phase due primarily to the presence of the high-speed phase modulator, resulting in an ambiguous phase offset.
Several of the deficiencies of the apparatus disclosed in commonly-owned U.S. Pat. No. 5,218,424 are addressed in copending U.S. patent application Ser. No. 08/38/232, dated Jan. 31, 1995, entitled "Interferometer and Method for Measuring the Distance of an Object Surface with Respect to the Surface of a Rotating Disk". The principle improvements taught in this copending Patent Application are the following: incorporation of compensation beams that permit high-speed operation without suffering from unwanted details concerning the variations in volume, surface profile and orientation of the disk; a high-speed phase measuring system; and a method and means for efficiently sampling the phase at a plurality of points corresponding to various positions on the slider surface at very high speed. Despite these advantages, the apparatus in this copending U.S. patent application Ser. No. 08/38/232 is still sensitive to some sources of phase drift, such as air turbulence and mechanical motion of the optical elements. Most seriously, it is excessively complicated and expensive.
In addition to the aforementioned significant disadvantages of known methods of flying height testing, another difficulty suffered by all prior art methods is the phase change that occurs at the slider surface upon reflection. The phase change can easily be misinterpreted as a change in flying height, resulting in errors as large as 20 nm. To correct for this effect, we must know the phase change exactly, using a priori knowledge of the complex index of refraction of the material. If the OFHT uses multiple wavelengths, then the complex index of refraction must be independently measured at each one of these wavelengths. See for example, the article entitled "Interferometric measurement of disk/slider spacing: The effect of phase shift on reflection," by C. Lacey, R. Shelor, A. Cormier (IEEE Transaction on Magnetics). The need for an independent measurement of the index of refraction places a significant burden on all prior-art methods and means for optical flying-height testing.
Most often, the index of refraction is measured by a separate instrument known in the art as an ellipsometer. Ellipsometers analyze the change in polarization of a beam reflected at an oblique from the surface of the material being tested. The geometry of this measurement is very different from all prior art flying-height measurement geometries, so a completely separate instrument is needed. According to the article "ellipsometry, a century old new technique," by R. F. Spanier (Industrial Research, September, 1975), an ellipsometer is an assembly of polarizers, retardation plates and detectors designed to measure the change in polarization of a light beam incident upon a test surface at an oblique angle. Ellipsometers provide information about the complex index of refraction of a surface, and can also provide information about multilayered media, such as thin films deposited on a substrate. Commercial ellipsometers include those manufactured by Gaertner Scientific Corporation in Chicago, Ill. Although the prior art provides several examples of experimental ellipsometers, including several designed for high-speed mesurement of the complex index of refraction, none of these instruments are designed for optical flying-height testing or like measurements of small gaps. Therefore, the determination of the complex index of refraction is a separate task from the gap measurement.
For example, in an article entitled "an automated scanning ellipsometer," by T. Smith (Surface Science 56, 212-220 (1976)), there is described an ellipsometric apparatus that has no moving parts. The light is incident upon a test surface at an oblique angle. Two beam splitters are disposed to sample spatially disparate portions of the reflected beam. One of the beam splitters separates out the polarizations perpendicular and parallel to the plane of incidence, whilst the other is placed at a 45.degree. angle with respect to the plane of incidence. The various beams are measured by photodetectors, and data processing based on a system of three mathematical formulas provides the appropriate ellipsometric parameters for characterizing the surface under test. Although the disclosed apparatus can function at high speed, it is unsuited to the task of measuring small gaps such as those encountered in optical flying height testing. A significant problem is the spatial sampling of the beam, which introduces errors and prevents its use with focused beams, even with the introduction of additional lenses and like focusing elements. Further, no method is taught in the article for high-speed, high precision measurement of the distance between two surfaces, one of which is on a substantially transparent element. Finally, no method is taught for determining the complex index of refraction of a surface separated from another surface by a small gap, such as a slider ABS flying in close proximity to a transparent disk. Therefore, the apparatus described by Smith does not solve the problem of independent measurement of the gap and of the complex index of refraction.
Some high-speed ellipsometers use heterodyne interferometry to generate signals. In an article entitled "interferometric ellipsometry" by H. F. Hazebroek and A. A. Holscher (J. Phys. E: Sci. Instrum. 6, 822-6 (1973)) there is described a specialized double-pass ellipsometer based on a Michelson interferometer and a scanning retroreflector. The test surface is placed together with a plane mirror in one arm of the interferometer, and the retroreflector is in the other arm. The scanning retroreflector generates beat-frequency signals for two orthogonal polarizations in the interferometer. These signals are measured photoelectrically, and their relative phase and amplitude provide the appropriate ellipsometric constants. A somewhat different arrangement for interferometic ellipsometry is described by C. Lin, C. Chou, and K. Chang in an article entitled "Real time interferometric ellipsometry with optical heterodyne and phase lock-in techniques." This article discloses an optical heterodyne technique involving a Mach-Zehnder geometry and two acousto-optic modulators to generate the beat-frequency signal. However, neither one of these heterodyne ellipsometers are designed for high-speed, high precision measurement of the distance between two surfaces, one of which is on a substantially transparent element. Also, no method is taught in these articles for determining the complex index of refraction of a surface separated from another surface by a small gap, such as a slider ABS flying in close proximity to a transparent disk. Therefore, neither the apparatus described by Hazebroek and Holscher nor that described by Lin solves the problem of independent measurement of the gap and of the complex index of refraction.
Yet another form of interferometric ellipsometer employs a Zeeman laser. The type disclosed in an article by L. Singher, A. Brunfeld and J. Shamir entitled "ellipsometry with a stabilized Zeeman laser" involves manual rotation of an analyzer, and so is not strictly a high-speed device. The type disclosed in U.S. Pat. No. 4,762,414 to G. Grego generates heterodyne signals, but is only capable of measuring one polarization at a time, and suffers from the additional difficulty that the reference leg of the interferometer must be protected from mechanical and thermal effects. These apparatus are therefore unsuited to the high-speed, high precision measurement of the distance between two surfaces, such as is required for optical flying-height testing.
A particularly unusual form of interferometric ellipsometer involving multiple passes across the test surface is described in the article "interference ellipsometer" by D. P. Pilipko and I. P. Pugach (Instruments and Experimental Techniques 26(4) 951-952 (1984)). The disclosed apparatus is based on a scanning Fabry-Perot interferometer. The unusual multiple-pass geometry simplifies some stages of signal processing for extracting ellipsometric data. However, the apparatus and method are particularly unsuited to the measurement of the distance between two surfaces, such as is required for optical flying-height testing.
Some forms of ellipsometer are only suited to a particular task or kind of surface. For example, U.S. Pat. No. 5,170,049 describes a specialized ellipsometer for the sole purpose of measuring the thickness of a chromic oxide coating on a chromium layer on a substrate. Another example of a specialized interferometric ellipsometer is disclosed in European Patent No. EP0075684 A1 to J.-C. Chastang, W. W. Hildenbrand and M. Levanoni. This system measures only the strength of two orthogonal polarizations and is therefore highly sensitive to small measurement errors and does not function at all at an incident angle of 45.degree.. Clearly neither one of these prior art techniques suggests a solution to the problem of small gaps such as are encountered in optical flying height testing, nor do they suggest a method for avoiding independent measurement of the flying height and of the complex index of refraction of the slider ABS.
It may be concluded, therefore, that the prior art does not provide any method or means of measuring flying height without relying on a separate measurement of the complex index of refraction. Therefore a principle difficulty in OFHT's and like gap-measuring instruments is that the determination of the complex index of refraction is an independent task from the gap measurement, involving an ellipsometer or like apparatus. This requirement significantly contribues significant cost and complexity to the measurement of small gaps, while at the same time it reduces the level of confidence in the result.
There is therefore an unmet need for an apparatus and method for high-speed, high precision measurement of the distance between two surfaces, such as is required for optical flying-height testing. Some of the difficulties which have occurred in the prior art include the inability to measure extremely narrow gaps, i.e. down to contact, the required use of multiple wavelengths and the associated complexity in the source and detector components, the sensitivity of some methods to air currents and mechanical distortions, and the need to determine the complex index of refraction independently with an ellipsometer. These disadvantages are overcome by the present invention.